A method and system for resourceful treatment of industrial wastewater containing fluorine and heavy metal ions

Abstract

This invention pertains to the field of pickling wastewater treatment in the steel industry. It discloses a method and system for the resource-based treatment of industrial wastewater containing fluoride and heavy metal ions. The method involves using sodium hydroxide to convert heavy metal and fluoride ions in the wastewater into hydroxides and fluoride complexes, which are then co-precipitated and separated from the aqueous phase. Sodium hydroxide is then used to convert the co-precipitated heavy metal and fluoride complexes into water-insoluble heavy metal hydroxides and water-soluble sodium fluoride. Calcium hydroxide is used to convert sodium fluoride in the water into calcium fluoride precipitate and water-soluble sodium hydroxide. The resulting NaOH aqueous solution is reused as part of the sodium hydroxide solution used in the preceding steps. Calcium hydroxide is then used to completely precipitate any remaining trace amounts of heavy metal and fluoride ions in the wastewater. The resulting wastewater then enters a biological treatment system for denitrification. This invention significantly reduces the amount of sodium hydroxide used and the amount of solid hazardous waste generated, alleviating the burden on biological treatment and lowering the overall cost of wastewater treatment.

Description

Technical Field

[0001] This invention belongs to, but is not limited to, the field of pickling wastewater treatment technology in the steel industry, and particularly relates to a method and system for the resource-based treatment of industrial wastewater containing fluoride and heavy metal ions. Background Technology

[0002] In stainless steel manufacturing, to remove the oxide scale from the surface of annealed steel, a mixed acid solution of nitric acid and hydrofluoric acid is generally used for surface cleaning. This process is called pickling or acid leaching, and it has been widely used both domestically and internationally due to its excellent cleaning effect. This mixed acid (pickling solution) can be reused repeatedly. When the pickling effect decreases, the pickling solution is treated as pickling wastewater before discharge. Currently, the most common method in China is to use lime slurry as a neutralizing and precipitating agent to treat stainless steel wastewater under strongly alkaline conditions. This is mainly because lime is widely available, lime slurry is inexpensive, and it has a good precipitation and removal effect on heavy metal ions such as chromium and fluoride ions in waste acid water. This process is stable, the water quality consistently meets standards, and operation and control are simple. However, in the daily operation of wastewater treatment plants in various enterprises, lime slurry is often added in excess to ensure that the effluent meets standards, resulting in a significant increase in the amount of sludge (more than 1.5 times the theoretical amount). Since this sludge is classified as solid hazardous waste, it is difficult to recycle and reuse, leading to high subsequent disposal costs. Chinese patent CN200710067749.8 discloses a method of using sodium hydroxide instead of lime as a neutralizing and precipitating agent. First, sodium hydroxide is added to the wastewater to adjust the pH to 9.0–9.5, precipitating heavy metal elements. Then, lime and other agents are added to remove the large amount of fluoride ions from the water. This method, to a certain extent, can reduce the amount of sludge compared to lime treatment, allowing for separate collection and treatment of heavy metal sludge and calcium fluoride, and the heavy metal sludge can be recycled. However, this method also has certain drawbacks: the cost of NaOH is much higher than that of lime, and the amount added is also larger, resulting in a higher overall cost for wastewater treatment than the process using lime as a neutralizing and precipitating agent; furthermore, the large amount of NaOH added leads to excessive salt (Na+ salt) content in the effluent after physicochemical treatment, affecting the activity of microorganisms and increasing the difficulty of subsequent biological treatment to remove NO3- ions from the effluent. Chinese patent CN200910101868.X discloses a method that first adjusts the pH of wastewater to 5-6 by adding lime slurry, then adds liquid alkali to adjust the pH to 9.0-9.5, forming heavy metal hydroxide precipitates to remove heavy metal ions from the wastewater. Next, hydrochloric acid, polyaluminum chloride, and lime slurry are added to precipitate and remove fluoride ions at pH 8-9. Finally, the wastewater is adjusted to neutral by adding hydrochloric acid before entering a biological treatment system for NO3- removal and discharge. The main drawback of this method is that while adding lime slurry at pH 5-6 can precipitate a large amount of heavy metal ions as hydroxides, it also produces a certain amount of calcium fluoride precipitate. At this point, the wastewater still contains a certain amount of unprecipitated heavy metal ions, as well as calcium and fluoride ions generated from the partial dissolution of calcium fluoride. Furthermore, after further adding liquid alkali to adjust the pH to 9.0-9.5, while heavy metal hydroxide precipitates are formed, dissolved calcium and fluoride ions inevitably precipitate as calcium fluoride.Therefore, the precipitation products of the above two-stage precipitation processes are mixtures of heavy metal hydroxides and calcium fluoride, which are actually still classified as hazardous solid waste. These mixed sludge residues containing fluorine and heavy metals are difficult to treat and recycle. Chinese patent CN201711277304.2 discloses a method in which NaOH is added to both the primary and secondary neutralization tanks to adjust the pH of the waste acid water to 5-6 and 7.5-8.5 respectively, to better control the pH value. Then, an ultra-micro separator is used for solid-liquid separation to obtain heavy metal sludge. The separated liquid is then treated with CaCl2, PAC, and PAM to remove fluoride. The purpose of this method is to achieve the separation and recycling of pure metal sludge and calcium fluoride sludge, achieving zero discharge of hazardous solid waste. However, when conducting verification experiments according to the method described in Chinese patent CN201711277304.2, it was found that when the pH of the mixed acid was adjusted to 5-8.5 with NaOH, colloidal precipitates such as Fe(OH)3 and Cr(OH)3 were easily generated. It was difficult to achieve solid-liquid separation using general methods (in Chinese patent CN201711277304.2, an ultra-micro separator was used to achieve this). Moreover, the colloidal precipitate contained a large number of fluoride ions that could not be removed by washing with water. These fluoride ions were mainly in the form of fluorine complexes of heavy metal ions (e.g., Na3FeF6, Na3CrF6) which were encapsulated and adsorbed in the colloidal precipitate of heavy metal hydroxides. This resulted in the inability to obtain relatively pure heavy metal sludge. Furthermore, the alkali used in neutralization and precipitation is all NaOH, which has disadvantages such as high reagent costs due to large NaOH usage and increased difficulty in biological treatment of the final process wastewater due to the high Na salt content. CaCl2 is used for defluorination, but the cost of CaCl2 is more than twice that of calcium hydroxide, which further increases the cost of defluorination reagents. In addition, a large amount of chloride ions will be introduced into the water, which will increase the burden on the final biological system for NO3- removal.

[0003] Based on the above analysis, the urgent technical problems that need to be solved in the existing technology are:

[0004] (1) In the existing technology, Ca(OH)2 is directly used to neutralize and precipitate wastewater under strong alkaline conditions. In order to ensure the treatment effect, an excessive amount of Ca(OH)2 needs to be added to remove heavy metal ions and fluoride ions in the wastewater. However, the amount of solid residue formed is far greater than the theoretical amount. Moreover, the solid residue is a mixture of hydroxides and fluorides of heavy metals, calcium hydroxide and calcium fluoride, which is a solid hazardous waste that is difficult to treat further.

[0005] (2) In existing patents, NaOH is directly used to neutralize and precipitate wastewater at pH 9.0–9.5 to remove heavy metal ions, followed by the use of Ca(OH)2 to remove fluoride ions. Although this reduces the amount of solid waste generated, the amount of NaOH used is too large, and the price of NaOH is significantly higher than that of Ca(OH)2, resulting in excessively high treatment costs. In addition, the large amount of NaOH added leads to an excessively high Na+ (salt) content in the separated liquid after solid-liquid separation, significantly increasing the burden and difficulty of subsequent biochemical denitrification (NO3-) treatment of the wastewater.

[0006] (3) In existing patents, the wastewater pH is adjusted to 5.0-6.0 by first adding Ca(OH)2 and then NaOH to adjust the pH to 9.0-9.5. This process precipitates heavy metal ions and fluoride ions in the wastewater in stages. Then, Ca(OH)2 is added to remove fluoride ions. Although this reduces the amount of NaOH used and the amount of solid slag generated, the solid slag formed in both stages is a mixture of heavy metal hydroxides and calcium fluoride, which is a solid hazardous waste that is difficult to treat further.

[0007] (4) In existing patents, NaOH is first used to adjust the pH of the wastewater to 8.5 to remove heavy metal ions, and then CaCl2 is used to remove fluoride ions. On the one hand, the resulting heavy metal hydroxide precipitate contains heavy metal fluoride complexes (Na3FeF6, Na3CrF6, etc.) that are difficult to remove by water washing, thus still forming solid hazardous waste; on the other hand, CaCl2, as a fluoride precipitant, is significantly more expensive than Ca(OH)2, and the amount of NaOH used to adjust the pH is still relatively large, increasing the cost of wastewater treatment agents, and the final process wastewater has a high content of Cl- and Na+, increasing the burden of further biochemical treatment.

[0008] In summary, existing technologies and patents for treating acidic wastewater containing heavy metal ions and fluoride ions all have at least one of the following shortcomings: the amount of waste residue generated after treatment is large; the waste residue is a mixture of heavy metal hydroxides and calcium fluoride, thus becoming solid hazardous waste; NaOH, used to neutralize the acidity of wastewater, is expensive and used in large quantities; CaCl2, used to precipitate fluoride ions, is relatively expensive; and the salt (Na+ and Cl-) content in the wastewater after removing heavy metal ions and fluoride ions is too high, increasing the burden and difficulty of further biochemical treatment. Summary of the Invention

[0009] To address the problems existing in the prior art, this invention provides a method and system for the resource-based treatment of industrial wastewater containing fluoride and heavy metal ions.

[0010] This invention is implemented as follows: a method for the resource-based treatment of industrial wastewater containing fluoride and heavy metal ions, the method specifically comprising:

[0011] S1: In the primary neutralization sedimentation tank, add NaOH to the industrial wastewater containing fluoride ions and heavy metal ions, adjust the pH to 5.3-5.8, stir and react for 15-30 minutes, and then perform solid-liquid separation on the resulting precipitate;

[0012] S2: Add water to the solid residue obtained in step S1 in a ratio of solid residue dry weight: water weight = 1:1 to 1:10 and mix it into a slurry. Then add NaOH to adjust the pH of the slurry to 10 to 11. Stir and react for 3 to 5 hours, and then perform solid-liquid separation.

[0013] S3: Add lime Ca(OH)2 to the separation liquid obtained in step S2, stir and react for 15-30 minutes, then separate the solid and liquid. The obtained solid is CaF2 containing only a small amount of unreacted Ca(OH)2 (<1Wt%), and the obtained separation liquid is NaOH solution, which can be reused in step S1 or step S2 as part of the alkali used to adjust the pH.

[0014] S4: Wash the solid residue obtained in step S2 with clean water at least twice and perform solid-liquid separation. The resulting heavy metal (mainly Cr, Fe, etc.) hydroxide mixture has low impurity content. The separation liquid after washing the solid residue contains a certain amount of NaOH. The separation liquid after the first washing of the solid residue is reused in the primary neutralization sedimentation tank, and the separation liquid after the second and subsequent washings of the solid residue is reused in the secondary neutralization sedimentation tank. Both are used as part of the alkali used to adjust the pH.

[0015] S5: The separated liquid obtained in step S1 and the separated liquid after the second and multiple washings of the solidified slag in step S4 are introduced into a secondary neutralization sedimentation tank. Ca(OH)2 is added to adjust the pH to 9.0-9.5, and 0.02‰ flocculant and 2‰ coagulant are added based on the mass of the separated liquid. The mixture is stirred and reacted for 15-30 minutes, followed by solid-liquid separation to obtain lime slag mainly containing Ca(OH)2 and a small amount of CaF2. The resulting separated liquid is adjusted to pH 7-9 with hydrochloric acid and then introduced into the biochemical system for denitrification (NO3). - )deal with.

[0016] Furthermore, in step S4, the solid residue is washed with clean water, and the pH of the solid-liquid separation liquid after the first washing of the solid residue is ≥10, and the pH of the solid-liquid separation liquid after the second or more washings of the solid residue is 7.0 to 9.0.

[0017] Furthermore, the specific amount of clean water used during the second or subsequent washing of the solid slag, or the specific pH control value of the separation liquid after washing the solid slag, is based on the removal of fluoride and sodium impurities from the final solid slag after washing. In other words, this means that the final heavy metal hydroxide slag meets the requirements for the minimum impurities in the slag that enable resource recycling.

[0018] Furthermore, the weight ratio of Ca(OH)2 added in step S3 to the fluorine content in the separation liquid is generally 1:1.0 to 1:3.0, so that the fluorine content in the separation liquid obtained after solid-liquid separation is relatively high. - The ion content is preferably less than 200 mg / L. The coagulant is composed of one or more of polyaluminum ferric chloride, polyferric chloride, polyferric sulfate, and polyaluminum chloride. The flocculant is polyacrylamide (PAM).

[0019] Furthermore, if the industrial wastewater containing fluorine and heavy metal ions contains hexavalent chromium (Cr), 6+ If the chromium is reduced to trivalent chromium, a small amount of appropriate reducing agent (such as zero-valent iron powder, ferrous sulfate, etc.) needs to be added first.

[0020] Another objective of this invention is to provide a resource recovery system for industrial wastewater containing fluoride and heavy metal ions, the system specifically comprising:

[0021] The precipitation separation module is used to precipitate and separate heavy metal ions and fluoride ions from wastewater.

[0022] The impurity removal and purification module is used to process mixtures of heavy metal hydroxides and their fluorine-containing complexes to achieve impurity removal and purification of heavy metal hydroxides;

[0023] The complete precipitation module is used to completely precipitate the small amount of heavy metal ions and fluoride ions remaining in the wastewater using calcium hydroxide.

[0024] The denitrification module is used to introduce the generated wastewater into the biological treatment system for denitrification (NO3). - )deal with.

[0025] Based on the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solution to be protected by this invention are as follows:

[0026] First, by adding sodium hydroxide at a pH of 5.3 to 5.8, this invention can precipitate more than 90% of heavy metal ions (chromium, iron, etc.) in wastewater, mainly in the form of heavy metal hydroxides and their fluorine-containing complexes (Na3FeF6, Na3CrF6). Compared with the process of using NaOH at a pH > 9.0 to precipitate all heavy metal ions in wastewater, this invention effectively reduces the consumption and cost of NaOH reagents.

[0027] This invention precipitates heavy metal hydroxides and their fluorine-containing complexes obtained by treating them with NaOH at pH = 10-11, converting fluoride ions in the precipitate into water-soluble sodium fluoride, thereby obtaining high-purity heavy metal hydroxides that can be recycled.

[0028] This invention converts the obtained NaF solution into high-purity calcium fluoride that can be recycled and recycled, as well as a NaOH solution, by adding Ca(OH)2, which significantly reduces the amount of NaOH used and the cost of reagents in the entire wastewater treatment process.

[0029] This invention converts the small amount of heavy metal ions and fluoride ions remaining in wastewater, which has already had most of its heavy metal ions and fluoride ions removed, into lime slag (mainly containing Ca(OH)2, CaF2 and heavy metal hydroxides) by adding Ca(OH)2, which significantly reduces the amount of solid hazardous waste generated and the salt content in the wastewater that ultimately enters the biochemical treatment system.

[0030] This invention significantly reduces the amount of NaOH added during wastewater treatment, substantially reduces the amount of solid hazardous waste generated, effectively alleviates the burden on biochemical treatment of wastewater, and achieves resource-based treatment of wastewater at a lower cost.

[0031] Second, the economic benefits of industrializing chromium-containing industrial sludge treatment services.

[0032] This invention addresses the treatment needs of chromium-containing industrial sludge by optimizing wastewater treatment processes in the stainless steel industry, thereby achieving the resource utilization of sludge. In a 500,000-ton-per-year stainless steel wastewater treatment project, the sludge volume was reduced to 8,706.08 tons / year through the technological upgrades of this invention, of which 4,880.84 tons of sludge can be recycled. The total treatment cost decreased from 20.6272 million yuan using traditional technologies to 17.788 million yuan, saving the waste-generating unit 2.8392 million yuan annually. Charging a 10% technical service fee based on the cost savings can generate an annual net profit of 283,900 yuan, while simultaneously promoting the industrialization of supporting services for waste disposal projects.

[0033] This invention enables the resource utilization of sludge in ceramic and paint pigments, significantly reducing the procurement of traditional raw materials (such as iron oxide red and chromium oxide green) and substantially lowering production costs. Taking one ton of black pigment as an example, the traditional process costs 4940 yuan, while the technology of this invention reduces the production cost to 2760 yuan, saving 44.12% per ton. Simultaneously, it solves the problem of harmless sludge treatment, providing pigment manufacturers with a more competitive solution.

[0034] Traditional treatment processes produce high-fluoride sludge that causes severe corrosion to ceramic kilns and related equipment, resulting in annual maintenance costs as high as 40 million yuan. This invention reduces the fluoride content in the sludge to below 3%, significantly reducing corrosion of kiln bricks, flue gas ducts, and waste heat boilers. This extends the equipment maintenance cycle from 3 months to approximately 2 years, reduces maintenance costs to a normal range, significantly lowers production costs, and extends equipment lifespan, solving the technical bottleneck that makes high-fluoride sludge unsuitable for production.

[0035] Thirdly, the prior art CN 117964162 A discloses a method for treating low-concentration fluoride-containing wastewater. Through multiple water washes, the fluoride content in the precipitation slag can be effectively removed (dry basis F < 5%). However, for high-concentration fluoride-containing wastewater (F > 5000 mg / L), the fluoride content in the generated precipitation slag significantly increases (15% < F < 30%). The effect of water wash treatment is limited, and it is unable to effectively reduce the fluoride content in the slag. The high-fluoride precipitation slag has special physical and chemical properties during the treatment process, especially showing significant corrosiveness under high-temperature conditions, which may cause serious damage to high-temperature kiln bricks and flue gas pipelines. This corrosiveness limits the applicability of existing high-temperature treatment technologies and also poses higher requirements for the safe treatment of high-fluoride materials. The resource utilization of high-fluoride precipitation slag is an important research direction in the field of wastewater treatment. However, the high fluoride content not only affects the processability of the materials but may also cause equipment wear and environmental risks, becoming the main obstacle in the process of resource utilization. Therefore, the prior art lacks an effective treatment method for high-fluoride materials. With the increasing concentration of fluoride in industrial wastewater, the existing low-concentration wastewater treatment technologies are difficult to meet the requirements. Especially when treating high-concentration fluoride-containing wastewater, the high fluoride content in the slag poses a threat to the safety of treatment equipment and subsequent processes. There is an urgent need for an innovative treatment method for high-fluoride precipitation slag to effectively control its fluoride content.

[0036] To address the above challenges, there is an urgent need to develop a treatment technology specifically for high-fluoride materials. This technology should be able to reduce the fluoride content of the materials while alleviating their corrosiveness and ensuring the safety and sustainability of the subsequent resource utilization process. This will provide reliable technical support for the treatment and resource utilization of high-concentration fluoride-containing wastewater and promote the development of the wastewater treatment field. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 is the flow chart of the resource treatment method for industrial wastewater containing fluoride and heavy metal ions provided by the embodiment of the present invention;

[0038] Figure 2 is the basic process route diagram provided by the embodiment of the present invention;

[0039] Figure 3 is the module diagram of the resource treatment system for industrial wastewater containing fluoride and heavy metal ions provided by the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0040] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the following further describes the present invention in detail with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0041] Such as Figure 1 、 Figure 2 As shown in the figure, this invention provides a method for the resource-based treatment of industrial wastewater containing fluoride and heavy metal ions, the method specifically including:

[0042] S1: In the primary neutralization sedimentation tank, add NaOH to the industrial wastewater containing fluoride ions and heavy metal ions, adjust the pH to 5.3-5.8, stir and react for 15-30 minutes, and then perform solid-liquid separation on the resulting precipitate;

[0043] S2: Add water to the solid residue obtained in step S1 in a ratio of solid residue dry weight: water weight = 1:1 to 1:10 and mix it into a slurry. Then add NaOH to adjust the pH of the slurry to 10 to 11. Stir and react for 3 to 5 hours, and then perform solid-liquid separation.

[0044] S3: Add lime Ca(OH)2 to the separated liquid obtained in step S2, stir and react for 15-30 minutes, and then separate the solid and liquid. The obtained solid is CaF2 containing only a small amount of unreacted Ca(OH)2 (<1Wt%), which can be widely used in industries such as cement, ceramics, glass and steel. The obtained separated liquid is NaOH solution, which can be reused in step S1 or step S2 as part of the alkali used to adjust the pH.

[0045] S4: Wash the solid residue obtained in step S2 with clean water at least twice and perform solid-liquid separation. The resulting mixture of heavy metal (mainly Cr, Fe, etc.) hydroxides has low impurity content (F content <2.0 wt%, Na <1.0 wt%), and can be used as raw materials for ceramic glazes and metal smelting. The separation liquid after washing the solid residue contains a certain amount of NaOH. The separation liquid after the first washing of the solid residue is reused in the primary neutralization sedimentation tank, and the separation liquid after the second and subsequent washings of the solid residue is reused in the secondary neutralization sedimentation tank, all of which are used as part of the alkali used to adjust the pH.

[0046] S5: The separated liquid obtained in step S1 and the separated liquid after the second and multiple washings of the solidified slag in step S4 are introduced into a secondary neutralization sedimentation tank. Ca(OH)2 is added to adjust the pH to 9.0-9.5, and 0.02‰ flocculant and 2‰ coagulant are added based on the mass of the separated liquid. The mixture is stirred and reacted for 15-30 minutes, followed by solid-liquid separation to obtain lime slag mainly containing Ca(OH)2 and a small amount of CaF2. The resulting separated liquid is adjusted to pH 7-9 with hydrochloric acid and then introduced into the biochemical system for denitrification (NO3). - )deal with.

[0047] The working principle of the resource-based treatment method for industrial wastewater containing fluoride and heavy metal ions.

[0048] 1. Primary neutralization and precipitation: Preliminary precipitation and separation

[0049] In the primary neutralization sedimentation tank, sodium hydroxide (NaOH) is added to the industrial wastewater containing fluoride ions and heavy metal ions to adjust the pH of the wastewater to 5.3-5.8. This pH range is suitable for heavy metal ions (such as Cr). 3 +, Fe 3 +) Optimal conditions for hydroxide precipitation. Stirring for 15-30 minutes ensures complete reaction, separating heavy metals and fluoride ions from the wastewater as precipitates. Solid-liquid separation yields a solid residue and a pre-treated liquid. The solid residue contains fluorides and heavy metal hydroxides, which proceed to subsequent treatment steps.

[0050] 2. Solid residue purification: secondary slurry conditioning and deep separation

[0051] After adding water to the solid residue obtained in step S1 to form a slurry, the pH of the slurry is adjusted to 10-11 again using NaOH. Under these alkaline conditions, fluoride ions and impurities in the heavy metal hydroxides are further released and new precipitates are formed. The reaction is stirred for 3-5 hours to ensure complete reaction. Subsequently, solid-liquid separation is performed to obtain high-purity heavy metal hydroxides and a separation liquid containing fluorides. The heavy metal precipitate is further purified by subsequent washing, and its impurity content (such as fluorine and sodium) is significantly reduced, making it suitable for use in ceramic glazes or metal smelting.

[0052] 3. Fluorine resource utilization: Extraction and recycling of calcium fluoride

[0053] The separated liquid obtained in step S2 is further reacted by adding lime (Ca(OH)2), which promotes the combination of fluoride ions and calcium ions to form high-purity calcium fluoride (CaF2). The calcium fluoride precipitate is collected through solid-liquid separation. Its impurity content is low (the content of incompletely reacted Ca(OH)2 is <1%), and it can be directly used as a resource product in the cement, ceramics, glass, and steel industries. The resulting separated liquid is a NaOH solution, which can be recycled back to step S1 or S2 as an alkaline source for adjusting pH, reducing chemical consumption.

[0054] 4. Secondary neutralization and denitrification: final purification and safe emission

[0055] The separated liquid from steps S1 and S4 is introduced into a secondary neutralization sedimentation tank. The pH is adjusted to 9.0-9.5 by adding lime, and flocculants and coagulants are added simultaneously to promote impurity sedimentation, generating lime slag mainly containing Ca(OH)2 and a small amount of CaF2. This lime slag is collected and processed after solid-liquid separation. The resulting separated liquid is adjusted to neutral (pH 7-9) using hydrochloric acid to ensure it meets the requirements for biochemical treatment, and then enters the biochemical system for denitrification. The denitrification process removes residual nitrates (NO3). - It is converted into nitrogen gas, achieving deep purification of nitrogen pollutants and ultimately ensuring that wastewater meets discharge standards.

[0056] 5. System closed loop and resource utilization

[0057] This method, through multi-stage precipitation, separation, and resource utilization, minimizes fluoride ion and heavy metal pollution in wastewater, while converting byproducts (such as calcium fluoride and heavy metal hydroxides) into high-value-added industrial raw materials. The NaOH solution and purified water in the separated liquid are recycled within the system, reducing the consumption of fresh chemicals and water resources, thus forming an environmentally friendly, efficient, and economical closed-loop system for the resource utilization of industrial wastewater.

[0058] In step S1, a relatively low amount of NaOH is used at a relatively low pH (5.3-5.8) to precipitate and separate precipitated heavy metal ions in wastewater as heavy metal hydroxides. The resulting heavy metal hydroxides are then further purified with NaOH to remove any entrained fluoride ions. This is because in wastewater containing heavy metal ions (mainly Fe...)... 3+ and Cr 3+ When NaOH is added to a solution containing fluoride ions, the following series of reactions occur:

[0059] Fe 3+ +3OH - →Fe(OH)3↓

[0060] Cr 3+ +3OH - →Cr(OH)3↓

[0061] Fe 3+ +3F - +xFe(OH)3→FeF3·xFe(OH)3↓

[0062] Cr 3+ +3F - +yCr(OH)3→CrF3·yCr(OH)3↓

[0063] 3Na + +3F - +FeF3·xFe(OH)3→Na3FeF6·xFe(OH)3↓

[0064] 3Na + +3F - +CrF3·xCr(OH)3→Na3CrF6·xCr(OH)3↓

[0065] Therefore, the heavy metal hydroxide precipitate obtained after adding NaOH contains fluoride ions in the form of heavy metal fluorine complexes. Without treatment, this heavy metal hydroxide will become hazardous solid waste. After solid-liquid separation and washing, the heavy metal hydroxide containing fluoride ions is further treated with NaOH under high alkalinity (pH = 10–11), resulting in the following reaction:

[0066] 3NaOH+Na3FeF6→6NaF+Fe(OH)3↓

[0067] 3NaOH+Na3CrF6→6NaF+Cr(OH)3↓

[0068] Because the generated NaF dissolves in water, the fluoride ions carried in the heavy metal hydroxide can be removed, thus transforming it from solid hazardous waste into a valuable raw material that can be utilized as a resource.

[0069] In step S3, Ca(OH)₂ is added to the separated liquid obtained after solid-liquid separation following NaOH treatment of heavy metal hydroxides, thereby converting the dissolved NaF in the separated liquid into NaOH and calcium fluoride. This is because the following reaction occurs between NaF and Ca(OH)₂ after the addition of Ca(OH)₂:

[0070] 2NaF + Ca(OH)₂ → 2NaOH + CaF₂↓

[0071] Since heavy metal hydroxides do not dissolve in a strongly alkaline environment (pH = 10-11) when treated with NaOH, the resulting solid-liquid separation liquid contains only NaF and almost no heavy metal ions. Therefore, the CaF2 precipitate obtained through the above reaction has high purity (no heavy metal ions) and can be utilized as a resource. The liquid phase after separating the CaF2 precipitate contains only NaOH, which can be reused as an alkaline solution to neutralize and adjust the pH value of the initial waste acid water. Therefore, the above alkaline treatment process for defluorination of heavy metal hydroxides does not increase the amount of NaOH used in the entire wastewater treatment process.

[0072] In step S5, Ca(OH)₂ is further added to the separated liquid obtained after precipitation of heavy metal hydroxides with NaOH. This causes the residual heavy metal ions and fluoride ions in the wastewater to precipitate out completely through the formation of lime slag (mainly containing Ca(OH)₂, heavy metal hydroxides, and calcium fluoride) at a pH of 9.0–9.5. Since over 90% of the heavy metal ions and fluoride are removed in the form of precipitation during NaOH neutralization and precipitation, the amount of lime slag obtained during further wastewater treatment with Ca(OH)₂ is significantly reduced compared to existing technologies. Furthermore, the Na salt content in the wastewater after separating the lime slag is greatly reduced, facilitating further biochemical treatment.

[0073] The heavy metal hydroxides (free of fluoride and sodium ions) and calcium fluoride (free of heavy metal ions) obtained from wastewater treatment have high purity and can be recycled and reused. For example, the former can be used as raw materials for ceramic glazes or steelmaking smelting, while the latter can be widely used as raw materials for the production of glass, ceramics and cement.

[0074] In step S4, the solid residue is washed with clean water. After the first washing of the solid residue, the pH of the solid-liquid separation liquid is ≥10, and after the second or subsequent washings of the solid residue, the pH of the solid-liquid separation liquid is 7.0 to 9.0.

[0075] The specific amount of clean water used during the second or subsequent washing of the solid slag, or the specific pH control value of the separation liquid after washing the solid slag, is based on the removal of fluoride and sodium impurities from the final solid slag after washing. In other words, this means that the final heavy metal hydroxide slag meets the requirements for the minimum impurities in the slag that allow for resource recycling.

[0076] The weight ratio of Ca(OH)2 added in S3 to the fluorine content in the separation liquid is generally 1:1.0 to 1:3.0, so that the fluorine content in the separation liquid obtained after solid-liquid separation is high. - The ion content is preferably less than 200 mg / L. The coagulant is composed of one or more of polyaluminum ferric chloride, polyferric chloride, polyferric sulfate, and polyaluminum chloride. The flocculant is polyacrylamide (PAM).

[0077] If the industrial wastewater containing fluoride and heavy metal ions (such as stainless steel mixed acid pickling wastewater) contains hexavalent chromium (Cr) 6+ If the chromium is reduced to trivalent chromium, a small amount of appropriate reducing agent (such as zero-valent iron powder, ferrous sulfate, etc.) needs to be added first.

[0078] like Figure 3 As shown in the figure, an embodiment of the present invention provides a resource recovery system for industrial wastewater containing fluoride and heavy metal ions, specifically comprising:

[0079] The precipitation separation module is used to precipitate and separate heavy metal ions and fluoride ions from wastewater.

[0080] The impurity removal and purification module is used to process mixtures of heavy metal hydroxides and their fluorine-containing complexes to achieve impurity removal and purification of heavy metal hydroxides;

[0081] The complete precipitation module is used to completely precipitate the small amount of heavy metal ions and fluoride ions remaining in the wastewater using calcium hydroxide.

[0082] The denitrification module is used to introduce the generated wastewater into the biological treatment system for denitrification (NO3). - )deal with.

[0083] (1) Precipitation separation module: efficiently removes heavy metals and fluoride ions

[0084] This system utilizes a precipitation separation module to separate heavy metal ions and fluoride ions from wastewater using chemical precipitation. By adding an appropriate amount of precipitant (such as sodium hydroxide or lime slurry), heavy metal ions in the wastewater form hydroxide precipitates, while fluoride ions form insoluble fluoride precipitates. This module effectively reduces the concentration of heavy metals and fluoride ions in the wastewater, providing stable water quality conditions for subsequent treatment and ensuring the foundation for treatment efficiency and resource utilization.

[0085] (2) Purification module: Optimization of purification of heavy metal hydroxides

[0086] Building upon precipitation separation, the purification module further refines the mixture of heavy metal hydroxides and their fluorine-containing complexes. Through multi-stage filtration and chemical cleaning, impurities in the mixture are removed, yielding high-purity heavy metal hydroxides. The purified heavy metal precipitate can be used for subsequent resource utilization, such as in the preparation of pigments or other industrial raw materials, while significantly reducing waste and improving economic efficiency and environmental value.

[0087] (3) Complete sedimentation and denitrification treatment module: to achieve comprehensive purification

[0088] For the trace amounts of heavy metal ions and fluoride ions remaining in the wastewater, the complete precipitation module uses calcium hydroxide for further treatment to ensure complete precipitation of residual pollutants and significantly reduce the concentration of heavy metals and fluoride ions in the effluent. Subsequently, the wastewater enters the denitrification treatment module, where a biological system removes nitrate ions (NO3) from the wastewater. - The wastewater is converted into nitrogen or other harmless substances, further purifying the effluent. This dual treatment mode ensures that wastewater discharge meets environmental standards and achieves the harmless and resource-based treatment of industrial wastewater.

[0089] I. Specific application areas or related products of this invention.

[0090] The waste acid water used in each embodiment came from pickling wastewater samples containing heavy metal ions, hydrofluoric acid, and nitric acid from a large stainless steel enterprise in Guangdong Province. The water quality is shown in Table 1.

[0091] Table 1: Water quality of pickling waste acid samples (unit: mg / L)

[0092]

[0093] *The sample is an average mixture of three samples taken from a stainless steel company.

[0094] Example 1 (Comparative Example 1)

[0095] In a neutralization sedimentation tank, 59.4 kg of lime (Ca(OH)2) was added to 2000 L of waste acid water under stirring conditions. The pH of the slurry was measured to be 9.0. Then, 1.8 kg of polyaluminum chloride and 0.02 kg of polyacrylamide (PAM) were added. Solid-liquid separation was performed on the slurry, yielding 114.8 kg of solid sludge cake with a water content of 50%. 1.5 L of 6 mol / L hydrochloric acid was added to the resulting separated liquid to adjust the pH to 7.0. The composition of the separated liquid and the solid sludge was analyzed, and the results are shown in Table 2.

[0096] Table 2

[0097]

[0098] Comparing the data in Table 2 and Table 1, it can be seen that the addition of lime can effectively remove heavy metal ions and fluoride ions from wastewater, with removal rates of 99.69%, 99.97%, and 99.85% for Fe, Cr, and F, respectively. The resulting solid slag contains a large amount of heavy metal ions (Fe, Cr), Ca, and F, with a solid slag yield of 0.0574 kg. 固渣 / L 废水 This solid slag belongs to solid hazardous waste.

[0099] Example 2 (Comparative Example 2)

[0100] In a neutralization sedimentation tank, 41.36 kg of solid NaOH was added to 2000 L of waste acid water under stirring conditions. The pH of the slurry was measured to be 9.5. Then, 1.8 kg of polyaluminum chloride and 0.02 kg of polyacrylamide were added. Solid-liquid separation was performed on the slurry, yielding 30.17 kg of a metal cake containing 50% water. The solid and liquid components were analyzed. 32.37 kg of lime was added to the resulting 1980 kg of separated liquid to remove F. -While stirring, 0.02 kg of polyacrylamide was added to perform solid-liquid separation of the slurry, yielding 47.6 kg of calcium fluoride sludge cake with a water content of 50%. 13 L of 6 mol / L hydrochloric acid was added to the separation liquid to adjust the pH to 7.0. The composition of the separation liquid and the solid residue was analyzed, and the results are shown in Table 3.

[0101] Table 3

[0102]

[0103] Comparing the data in Table 3 and Table 2, it can be seen that adjusting the pH of the waste acid to 9.5 with NaOH can essentially remove heavy metal ions from the wastewater, and also yields a metal slag with relatively low levels of sodium and fluorine impurities that can be utilized. After further removing fluorine from the separation liquid with lime, the resulting calcium fluoride slag also has very low heavy metal content and high calcium fluoride content, making it valuable for resource utilization. Therefore, the advantage of using NaOH to neutralize waste acid is that it can significantly reduce the volume and quality of the generated sludge. However, Table 3 shows that the final effluent after solid-liquid separation of wastewater treated with NaOH introduces Na... + The levels are very high, which will affect the subsequent deNO3 removal process in the biochemical system. - This increases the difficulty. The comparison of reagent and treatment costs between Example 2 and Example 1 is shown in Table 4 below:

[0104] Table 4

[0105]

[0106] As shown in Table 4, treating waste acid water with NaOH to neutralize acid and precipitate heavy metal ions requires a significant amount of NaOH reagent. Furthermore, since the unit cost of NaOH reagent is much higher than that of lime reagent, while adding NaOH can substantially reduce or even completely utilize sludge, it also significantly increases the cost of NaOH reagent. The overall cost may be slightly higher than that of the lime treatment process.

[0107] Example 3

[0108] Step 1

[0109] 2000L of waste acid water was introduced into a primary neutralization sedimentation tank, and 34.36kg of solid NaOH was added and stirred until homogeneous. The pH value of the waste acid water was measured to be 5.51. After reacting for 20 minutes, the reaction solution was subjected to solid-liquid separation, yielding 26.9kg of sludge cake with a water content of 50%. The test results are shown in Tables 5 and 6.

[0110] Table 5

[0111]

[0112] Comparative analysis of the data in Table 5 and Example 2 shows that the use of this patented invention with NaOH only adjusts the pH of the waste acid water to 5.5, thus reducing the amount of NaOH required. Furthermore, at pH 5.5, it can remove over 90% of heavy metal ions (Fe, Cr) and over 85% of F. - It is separated from the wastewater in the form of precipitation, and the Na ions in the filtrate are significantly reduced.

[0113] Table 6

[0114]

[0115] Analysis of the data in Table 6 shows that, using the patented technology of this invention, the precipitate obtained by adding NaOH at pH 5.5 mainly contains Fe, Cr, F, and Na, with a F / Na molar ratio of approximately 2.5. In contrast, the known F / Na molar ratio in the heavy metal fluorine complex Na3MF6 (M = Fe, Cr, etc.) is 2.0, and the F / Na molar ratio in the heavy metal fluoride MF3 (M = Fe, Cr, etc.) is 3.0. Therefore, the precipitate is composed of heavy metal hydroxides, fluorides, and fluorine-containing complexes.

[0116] Step 2

[0117] In the alkaline washing residue mixing tank, 26.9 kg of mud cake (13.45 kg of dry residue) obtained after solid-liquid separation at pH = 5.5 in step 1 and 135 kg of water (tap water) were first added, and stirring was started. Then, 1.3 kg of NaOH (solid) was added, and the pH of the slurry was measured to be 10.7. Stirring was continued for 4.0 hours to perform solid-liquid separation of the slurry. The composition of the obtained separated liquid (NaF liquid) and solid residue (first alkaline washing residue) was analyzed, and the results are shown in Table 7.

[0118] Table 7

[0119]

[0120] Based on the data in Table 7 and a comparative analysis with the data in Tables 5 and 6, after one alkaline washing of the slag, the ratio of Fe to Cr content in the solid slag did not change significantly, while the ratio of the sum of Fe and Cr content to Na and F content increased significantly. Furthermore, the separated liquid contained a large amount of F and Na, and only a small amount of Fe and Cr. This indicates that after one alkaline washing of the slag, F and Na were effectively eluted from the solid slag and entered the eluent.

[0121] Step 3

[0122] 127.5 kg of the filtrate (NaF solution) obtained in step 2 was mixed with 3.5 kg of lime (Ca(OH)2) in a stirred tank (NaF displacement reaction tank) for 20 minutes. Solid-liquid separation was then performed using a filtration device. The composition of the separated liquid is shown in Table 8. The separated liquid (NaOH alkali solution) was added to the solid residue obtained in step 2 after the first alkali washing, and a second alkali washing was performed in the alkali washing residue stirred tank. Solid-liquid separation was then performed again. The composition of the resulting solid residue (secondary alkali washing residue) and separated liquid was analyzed, and the results are shown in Table 9. 126 kg of the separated liquid (NaOH) was refluxed into the primary neutralization sedimentation tank to neutralize and treat the waste acid.

[0123] Table 8

[0124]

[0125]

[0126] Based on the data in Table 8 and a comparison with the data in Table 7, it can be seen that after the second alkaline washing of the residue using recycled NaOH alkaline solution, the resulting separated liquid contains a large amount of F. - and Na + Therefore, the separated liquid is essentially still a NaF solution. Fluoride ions in the resulting solid residue (secondary alkali washing residue) are essentially removed, and the sodium ion content is also significantly reduced to a low level (at this point, sodium and fluorine in the solid residue mainly exist in the form of NaOH and NaF).

[0127] 7.2 kg of lime (Ca(OH)2) was added to 151 kg of the NaF solution obtained in step 3 in a stirred tank, and the mixture was stirred for 20 minutes to perform solid-liquid separation. The separated liquid was then sent to a primary neutralization and precipitation tank to neutralize and treat the waste acid. The composition of the obtained separated liquid was analyzed, and the results are shown in Table 9.

[0128] Table 9

[0129]

[0130] Analysis of the data in Table 9 shows that the separation solution contains a high concentration of Na ions, while F... - The content of heavy metal ions is very low. Furthermore, the pH values ​​of the primary and secondary separation solutions (recycled alkali solutions) were measured to be 12.7 and 13.6, respectively. Therefore, the above separation solutions are essentially NaOH solutions and can be added to the primary neutralization sedimentation tank as recycled alkali solutions for the neutralization of waste acid.

[0131] The composition of the solid residue obtained by adding lime Ca(OH)2 to the NaF solution in step 3 above was analyzed, and the results are shown in Table 10.

[0132] Table 10

[0133]

[0134]

[0135] The data in Table 10 show that the solid slag mainly contains Ca and F, with the Ca / F molar ratio being greater than the theoretical value of CaF2 = 0.5 in CaF2. This indicates that the solid slag is mainly composed of CaF2 and Ca(OH)2.

[0136] The solid residue composed of CaF2 and Ca(OH)2 was further purified to remove unreacted lime Ca(OH)2 and other impurities. 0.3 L of 6 mol / L dilute hydrochloric acid and 25 kg of water were added to 24.3 kg of solid residue (50% water content). After stirring thoroughly, the pH of the slurry was measured to be 7.9. The slurry underwent solid-liquid separation, and the separated liquid was sent to a secondary neutralization sedimentation tank for further treatment. The composition of the obtained solid residue was analyzed, and the results are shown in Table 11.

[0137] Table 11

[0138]

[0139] As shown in Table 11, the solid residue obtained after purification mainly contains Ca and F, with very few other impurities. Furthermore, the Ca / F molar ratio is close to 0.5, indicating that the solid residue is CaF2. The purity of this CaF2 is >95%, and the content of heavy metal elements [Cr, Ni, Mn] is less than 0.01%, indicating high resource recycling value.

[0140] Step 4

[0141] 200 kg of tap water was added to the 31.1 kg solid residue (sludge cake) obtained after the second alkaline washing in step 3 in a mixing tank. After stirring, the pH of the sludge was tested to be 10.5. After stirring for 20 minutes, solid-liquid separation was performed. A portion of the 201 kg of separated liquid (NaOH) was used as the alkali (NaOH) for washing the residue in step 2, and the remainder was sent to a primary neutralization sedimentation tank to neutralize and treat the waste acid. The composition of the separated liquid after water washing of the solid residue was analyzed, and the results are shown in Table 12.

[0142] Table 12

[0143]

[0144] As can be seen from the data in Table 12, the Na content in the separated liquid after washing the solid residue with water is high. + The content is relatively high, F -The content of certain substances is relatively low, while the content of heavy metals and other impurities is very low. Tests show that the alkalinity of the separated liquid after washing the solid slag is relatively strong (pH≥10.0). Therefore, the separated liquid after washing the solid slag can also be used as a NaOH recycling alkaline solution, and can be used as a neutralization and precipitation agent for waste acid in the primary neutralization sedimentation tank.

[0145] In a mixing tank, 200 kg of clean water was added to the solid-liquid separation residue (28.5 kg). After stirring and washing for 20 minutes, the pH of the slurry was tested and found to be 7.7. The slurry underwent solid-liquid separation, and the separated liquid entered a secondary neutralization sedimentation tank. The composition of the obtained solid residue and separated liquid was analyzed, and the results are shown in Table 13.

[0146] Table 13

[0147]

[0148] Analysis of the data in Table 13 shows that the solid slag obtained after the secondary washing (final washing slag, metal hydroxide slag) mainly contains Fe and Cr. The contents of the main impurities, fluorine and sodium, are reduced to less than 1%, and the contents of other impurities are also very low, which can meet the requirements for use as raw materials for ceramic glazes or for resource recycling in other industries. The Na content in the separation liquid after the secondary washing is low, and its pH value is less than 8.0. Due to the low alkalinity, it is not necessary to use it as a reuse alkali solution. It can enter the secondary neutralization sedimentation tank and be treated together with the intermediate wastewater to remove the small amount of heavy metal impurities and fluoride ions in the wastewater.

[0149] Step 5

[0150] The solid-liquid separation liquid obtained in step 1 and the separation liquid obtained after secondary water washing in step 4 were combined and fed into a secondary neutralization sedimentation tank. Lime (Ca(OH)2), 0.04 kg of flocculant (PAM), and 4 kg of polyaluminum ferric chloride coagulant (PAFC) were added and stirred until homogeneous. The pH of the mixture was measured to be 9.5. After standing for 1 hour, solid-liquid separation was performed. The filtrate was adjusted to pH 7.0 with 6 mol / L hydrochloric acid (2 L) and then introduced into a biochemical system for NO3 removal. - Processing. The composition of the obtained solid sludge (slag from the secondary neutralization sedimentation tank) and separated liquid (effluent from the secondary neutralization sedimentation tank) was analyzed, and the results are shown in Table 14.

[0151] Table 14

[0152]

[0153]

[0154] Analysis of the data in Table 14 shows that the slag from the secondary neutralization sedimentation tank mainly contains Ca and F, as well as small amounts of heavy metals, while the effluent from the secondary neutralization sedimentation tank contains very low levels of Ca, F, and small amounts of heavy metals. Besides NO3, the effluent from the secondary neutralization sedimentation tank also contains other heavy metals. - Aside from a relatively high content, all other components met the requirements of the relevant national standards, specifically the "Water Pollutant Discharge Standard for Iron and Steel Industry: GB13456-2012". NO3 in the effluent from the secondary neutralization sedimentation tank... - It is further processed through a biochemical system.

[0155] Example 4

[0156] Step 1

[0157] Waste acid water (2000L) was introduced into a primary neutralization sedimentation tank. First, 126kg of recycled alkali solution (NaOH) from step 3 of Example 3 and 47kg of recycled alkali solution (NaOH) from step 4 of Example 3 were added. Then, 21.17kg of solid NaOH was added, and the mixture was stirred until homogeneous. The pH of the waste acid water was measured to be 5.51. After stirring and reacting for 20 minutes, the reaction solution was subjected to solid-liquid separation, yielding 26.2kg of sludge cake with a water content of 50%. The composition of the separated liquid and the solid residue was analyzed, and the results are shown in Tables 15 and 16.

[0158] Table 15

[0159]

[0160] Comparative analysis of the data in Table 15 with Example 2 shows that, by using the primary sedimentation tank of this invention and reusing the recycled alkali solution (NaOH) from Example 3, the amount of NaOH used is significantly reduced compared to Example 2. The amount of NaOH added is reduced by 48.8% compared to Example 2. Simultaneously, at pH 5.5, over 90% of heavy metal ions (Fe, Cr) and over 86% of F are removed. - It is separated from the wastewater in the form of precipitation, and the Na ions in the filtrate are significantly reduced.

[0161] Table 16

[0162]

[0163] Analysis of the data in Table 16 shows that the precipitate obtained by adding NaOH at pH 5.5 using the patented technology of this invention is similar to the data in Table 6 of Example 3. It is also determined that the precipitate is composed of heavy metal hydroxides, fluorides and fluorine-containing complexes.

[0164] Step 2

[0165] In the alkaline washing residue mixing tank, 26.2 kg of mud cake (13.1 kg of dry residue) obtained after solid-liquid separation at pH = 5.5 in step 1 was first added. Then, 154 kg of recycled alkaline solution (NaOH) from step 4 of Example 3 was added, and stirring was started. At this time, the pH of the slurry was measured to be 10.9, and stirring was continued for 4.0 hours. The slurry was subjected to solid-liquid separation, and the composition of the obtained separated liquid (NaF solution) and solid residue (first-stage alkaline washing residue) was analyzed. The results are shown in Table 17.

[0166] Table 17

[0167]

[0168] Based on the data in Table 17 and a comparative analysis with the data in Tables 16 and 15, it can be seen that after one alkaline washing of the slag, the ratio of Fe to Cr content in the solid slag does not change significantly, while the ratio of the sum of Fe and Cr content to Na and F content increases significantly. Furthermore, the separated liquid contains a large amount of F and Na, and only a small amount of Fe and Cr. This indicates that after one alkaline washing of the slag, F and Na are effectively eluted from the solid slag and enter the eluent.

[0169] Step 3

[0170] 153 kg of the filtrate (NaF solution) obtained in step 2 was mixed with 4.2 kg of lime (Ca(OH)2) in a stirred tank (NaF displacement reaction tank) for 20 minutes. Solid-liquid separation was performed using a filtration device. The separated liquid (NaOH alkali solution) was added to the solid residue obtained in step 2 after the first alkali washing, and a second alkali washing was performed in the stirred tank. Solid-liquid separation was then repeated. The composition of the resulting solid residue (secondary alkali washing residue) and the separated liquid was analyzed, and the results are shown in Table 18.

[0171] Table 18

[0172]

[0173] Based on the data in Table 18 and a comparison with the data in Table 17, it can be seen that after the second alkaline washing of the residue using recycled NaOH alkaline solution, the resulting separated liquid contains a large amount of F. - and Na + Therefore, the separated liquid is essentially still a NaF solution. Fluoride ions in the resulting solid residue (secondary alkali washing residue) are essentially removed, and the sodium ion content is also significantly reduced to a low level (at this point, sodium and fluorine in the solid residue mainly exist in the form of NaOH and NaF).

[0174] In a stirred tank, 7.2 kg of lime (Ca(OH)2) was added to 151 kg of the NaF solution obtained in step 3, and the mixture was stirred for 20 minutes to perform solid-liquid separation. The separated liquid was then sent to a primary neutralization and precipitation tank to neutralize and treat the waste acid. The composition of the obtained separated liquid was analyzed, and the results are shown in Tables 5, 19, and 20.

[0175] Table 19

[0176]

[0177] As can be seen from the data in Table 19, the separation solution contains a high concentration of Na ions, while F - The content of heavy metal ions is very low. Furthermore, the pH of the separated liquid was tested to be 13.52. Therefore, the separated liquid is essentially a NaOH solution and can be added to the primary neutralization sedimentation tank as a recycled alkali solution for the neutralization of waste acid.

[0178] The composition of the solid residue obtained by adding lime Ca(OH)2 to the NaF solution in step 3 above was analyzed, and the results are shown in Table 20.

[0179] Table 20

[0180]

[0181] The data in Table 20 show that the solid slag mainly contains Ca and F, with the Ca / F molar ratio being greater than the theoretical value of CaF2 = 0.5 in CaF2. This indicates that the solid slag is mainly composed of CaF2 and Ca(OH)2.

[0182] The solid residue composed of CaF2 and Ca(OH)2 was further purified to remove unreacted lime Ca(OH)2 and other impurities. 0.3 kg of 1:1 dilute hydrochloric acid and 25 kg of water were added to 23.4 kg of solid residue (50% water content). After stirring thoroughly, the pH of the slurry was measured to be 8.0. The slurry underwent solid-liquid separation, and the separated liquid was sent to a secondary neutralization sedimentation tank for further treatment. The composition of the obtained solid residue was analyzed, and the results are shown in Table 21.

[0183] Table 21

[0184]

[0185] As shown in Table 21, the solid residue obtained after purification mainly contains Ca and F, with very few other impurities. Furthermore, the Ca / F molar ratio is close to 0.5, indicating that the solid residue is CaF2. The purity of this CaF2 is >95%, and the content of heavy metal elements [Cr, Ni, Mn] is less than 0.01%, indicating high resource recycling value.

[0186] Step 4

[0187] 200 kg of clean water was added to the 30.1 kg solid residue (sludge cake) obtained after the second alkaline washing in step 3 in a mixing tank. After stirring, the pH of the sludge was tested to be 10.2. After stirring for 20 minutes, solid-liquid separation was performed. A portion of the 201.5 kg of separation liquid (NaOH) was used as the alkali (NaOH) for washing the residue in step 2, and the remainder was sent to a primary neutralization sedimentation tank to neutralize the waste acid. The composition of the separation liquid after water washing of the solid residue was analyzed, and the results are shown in Table 22.

[0188] Table 22

[0189]

[0190] As can be seen from the data in Table 22, the Na content in the separated liquid after washing the solid slag with water is high. + The content is very high, F - The content of certain substances is relatively low, while the content of heavy metals and other impurities is very low. Tests show that the alkalinity of the separated liquid after washing the solid slag is relatively strong (pH≥10.0). Therefore, the separated liquid after washing the solid slag can also be used as a NaOH recycling alkaline solution, and can be used as a neutralization and precipitation agent for waste acid in the primary neutralization sedimentation tank.

[0191] 200 kg of clean water was added to the solid-liquid separation residue (28.3 kg) obtained in step 4 in a mixing tank. After stirring and washing for 20 minutes, the pH of the mud was tested and found to be 7.8. After solid-liquid separation, the composition of the separated solid residue and the separated liquid was analyzed, and the results are shown in Table 23.

[0192] Table 23

[0193]

[0194] Analysis of the data in Table 23 shows that the solid slag obtained after the secondary washing (final washing slag, metal hydroxide slag) mainly contains Fe and Cr. The contents of the main impurities, fluorine and sodium, are reduced to less than 1%, and the contents of other impurities are also very low, which can meet the requirements for use as raw materials for ceramic glazes or for resource recycling in other industries. The Na content in the separation liquid after the secondary washing is low, and its pH value is less than 8.0. Due to the low alkalinity, it is not necessary to use it as a reuse alkali solution. It can enter the secondary neutralization sedimentation tank and be treated together with the intermediate wastewater to remove the small amount of heavy metal impurities and fluoride ions in the wastewater.

[0195] Step 5

[0196] The solid-liquid separation liquid obtained in step 1 and the separation liquid obtained after secondary water washing in step 4 were combined and fed into a secondary neutralization sedimentation tank. Lime Ca(OH)2, 0.04 kg of flocculant (PAM), and 4 kg of polyaluminum ferric chloride coagulant (PAFC) were added and stirred until homogeneous. The pH of the mixture was measured to be 9.6. After standing for 1 hour, solid-liquid separation was performed. The pH of the filtered filtrate was adjusted to 7.0 using 6 mol / L hydrochloric acid (2 L). The composition of the obtained solid residue (slag from the secondary neutralization sedimentation tank) and separation liquid (effluent from the secondary neutralization sedimentation tank) was analyzed, and the results are shown in Table 24.

[0197] Table 24

[0198]

[0199] As can be seen from the data in Table 24, the slag from the secondary neutralization sedimentation tank mainly contains Ca and F, as well as small amounts of heavy metals, while the effluent from the secondary neutralization sedimentation tank has very low levels of Ca, F, and small amounts of heavy metals. Besides NO3, the effluent from the secondary neutralization sedimentation tank also contains... - Aside from a relatively high content, all other components met the requirements of the relevant national standards, specifically the "Water Pollutant Discharge Standard for Iron and Steel Industry: GB13456-2012". NO3 in the effluent from the secondary neutralization sedimentation tank... - It is further processed through a biochemical system.

[0200] This embodiment compares the cost per ton of reagent with that of the traditional lime method.

[0201] For the same batch of stainless steel pickling wastewater in the example, the commonly used process of directly adding lime (Ca(OH)2) to treat the wastewater in stainless steel enterprises was adopted. The cost and economic benefits of the technology of this invention compared with the traditional lime treatment process were compared. The main focus was on the cost of the dosage of chemicals consumed, the amount of sludge produced, and the disposal cost for treating more than 2 tons of the same batch of pickling wastewater. The results are shown in Table 25 below.

[0202] Table 25

[0203]

[0204]

[0205] The above analysis shows that, compared with the traditional lime neutralization process for wastewater treatment, the amount of hazardous solid waste requiring disposal is reduced by more than 95% using the technology of this invention. In treating fluoride-containing wastewater, this invention cleverly utilizes the alkaline washing process to first convert fluoride and sodium impurities (hexafluoride compounds) in the slag into NaF, which is then converted into NaOH using low-cost lime. This significantly reduces the consumption of NaOH reagents, thus addressing the problem of high reagent costs in wastewater treatment with NaOH. More importantly, it achieves a solid waste recycling rate of over 90%, separating and converting the originally hazardous mixed slag containing chromium and fluoride into valuable metal oxide slag and high-purity valuable calcium fluoride inorganic slag. This not only "reduces costs and increases efficiency, saves energy and reduces emissions, and promotes sustainable resource utilization and development," but also better aligns with the national development principles and requirements of "reduction, resource utilization, and harmlessness."

[0206] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention, and within the spirit and principles of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for the resource-based treatment of industrial wastewater containing fluoride and heavy metal ions, characterized in that, The method specifically includes: S1: In the primary neutralization sedimentation tank, add NaOH to the industrial wastewater containing fluoride ions and heavy metal ions, adjust the pH to 5.3-5.8, stir and react for 15-30 minutes, and then perform solid-liquid separation on the resulting precipitate; S2: Add water to the solid residue obtained in step S1 at a ratio of solid residue dry weight: water weight = 1:1 ~ 1:10 and mix into a slurry. Then add NaOH to adjust the pH of the slurry to 10 ~ 11, stir and react for 3-5 hours, and then perform solid-liquid separation. S3: Add lime Ca(OH)2 to the separation liquid obtained in step S2, stir and react for 15-30 minutes, and then separate the solid and liquid. The solid obtained is CaF2 containing only < 1 Wt% of unreacted Ca(OH)2. The separation liquid obtained is NaOH solution, which can be reused in step S1 or step S2 as part of the alkali used to adjust the pH. S4: Wash the solid residue obtained in step S2 with clean water at least twice and perform solid-liquid separation. The resulting heavy metal hydroxide mixture has low impurity content. The separation liquid after washing the solid residue contains a certain amount of NaOH. The separation liquid after the first washing of the solid residue is reused in the primary neutralization sedimentation tank, and the separation liquid after the second and subsequent washings of the solid residue is reused in the secondary neutralization sedimentation tank. Both are used as part of the alkali used to adjust the pH. S5: The separated liquid obtained in step S1 and the separated liquid after the second and multiple washings of the solid slag in step S4 are introduced into the secondary neutralization sedimentation tank. Ca(OH)2 is added to adjust the pH to 9.0-9.5, and 0.02‰ flocculant and 2‰ coagulant are added according to the mass of the separated liquid. The mixture is stirred and reacted for 15-30 minutes, and then solid-liquid separation is performed to obtain lime slag mainly containing Ca(OH)2 and a small amount of CaF2. The obtained separated liquid is adjusted to pH 7-9 with hydrochloric acid and then introduced into the biochemical system for denitrification treatment.

2. The method for resource-based treatment of industrial wastewater containing fluoride and heavy metal ions as described in claim 1, characterized in that, In step S4, the solid residue is washed with clean water. After the first washing of the solid residue, the pH of the solid-liquid separation liquid is ≥ 10, and after the second or subsequent washings of the solid residue, the pH of the solid-liquid separation liquid is 7.0 ~ 9.

0.

3. The method for resource-based treatment of industrial wastewater containing fluoride and heavy metal ions as described in claim 1, characterized in that, The specific amount of clean water used during the second or subsequent washing of the solid slag, or the specific pH control value of the separation liquid after washing the solid slag, is based on the removal of fluoride and sodium impurities from the final solid slag after washing. This ensures that the final heavy metal hydroxide slag meets the requirements for the minimum impurities in the slag for resource recycling.

4. The method for resource recovery treatment of industrial wastewater containing fluoride and heavy metal ions as described in claim 1, characterized in that, The weight ratio of Ca(OH)2 added in S3 to the fluorine content in the separation liquid is 1:1.0 ~ 1:3.0, so that the fluorine content in the separation liquid obtained after solid-liquid separation is... - The ion content is less than 200 mg / L, the coagulant is one or more of polyaluminum ferric chloride, polyferric chloride, polyferric sulfate, and polyaluminum chloride, and the flocculant is polyacrylamide (PAM).

5. The method for resource-based treatment of industrial wastewater containing fluoride and heavy metal ions as described in claim 1, characterized in that, If the industrial wastewater containing fluorine and heavy metal ions contains hexavalent chromium (Cr) 6+ If chromium is to be reduced to trivalent chromium, a small amount of reducing agent must be added first.

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